1. Field of the Invention
This invention relates to a process for the production of ammonium fatty acid hydroxyalkanesulfonates in which an ammonium hydroxyalkanesulfonate and a fatty acid containing 6 to 18 carbon atoms are condensed at elevated temperature.
2. Description of the Related Art
Fatty acid hydroxyalkanesulfonic acid salts, more particularly fatty acid isethionates in the C.sub.12-14 chain length range, are anionic surfactants with only minimal sensitivity to hardness, high foaming and wetting power and excellent compatibility with the skin. More particularly, they are distinguished by the fact that the skin can be cleansed without overly drying out. In addition, the soaps containing these compounds can even be used by people unable to tolerate typical high pH soaps. Accordingly, these compounds are used in cosmetic preparations and cleansing formulations.
In the main, only processes for the production of sodium salts of fatty acid hydroxyalkanesulfonic acids are known from the prior art.
European patent application EP-A-0 073 626 describes a process for the production of fatty acid isethionates. In this process, an aqueous sodium isethionate solution is initially introduced together with a catalyst, which is a combination of ZnO with a sulfonic acid, and the fatty acid is subsequently added. The condensation reaction is carried out with an excess of fatty acid. To obtain a product of high activity, the excess free fatty acid is removed from the reaction mixture on completion of the reaction.
EP-A-0 176 330, for example, describes a mild cleansing composition containing sodium isethionate and fatty acid soap in addition to fatty acid isethionate. To produce this composition, the principal components are combined in the form of an aqueous solution at 100.degree. to 110.degree. C. and are mixed in a mixer at a high shear rate. The mixture is then dried to the required moisture content and made up.
The processes mentioned above give high yields of, above all, the sodium salts of fatty acid hydroxyalkanesulfonic acids. However, the sodium salts produced have the disadvantage that they are very sparingly soluble in water and, accordingly, are unsuitable for clear liquid formulations. In contrast to the sodium salt, the ammonium salt is distinguished by good solubility in water. In addition, it is a very mild, high-foaming surface-active agent which has proved to be particularly kind to the skin. Unfortunately, the processes described above cannot be applied to the production of the corresponding ammonium salts. The ammonium salts differ from the sodium salts in their solubility in water, in their high foaming capacity and in their thermal instability. On account of the high foaming power of ammonium hydroxyalkanesulfonates, more particularly in aqueous solution, it is difficult to carry out the process at low fatty acid contents because the reaction mixture overfoams and becomes uncontrollable. Another difficulty is that the reaction temperature is near the decomposition point of the compound so that the reaction products can very easily become dark in color. There is no known process for the production of these ammonium salts on an industrial scale.
The problem addressed by the present invention was to provide a process which could even be used on an industrial scale for the production of ammonium fatty acid hydroxyalkanesulfonates without any process-related difficulties arising and which would enable ammonium fatty acid hydroxyalkanesulfonates to be obtained in high yields and in a light color.